Dimming your screen, turning off apps and avoiding unnecessary use can preserve cell phone battery for emergency use.
Communications can be vital during an emergency, so make sure you have your cell phone fully charged and, if the power goes out, only use it if necessary. Text messages can be quicker and take up less energy than calls but if you're keeping your phone on, turn off notifications (like the notices that pop up when you get email) and any unnecessary apps as well as WiFi, GPS and Bluetooth to preserve energy.
If you have a smartphone, dim the screen to its lowest possible level. And, as tempting as it may be to surf the web or access Facebook and Twitter, turn off data unless you need it because that, too, uses lots of energy. Also turn off vibrate mode -- that too uses extra energy. If you must leave data on, use the slowest (and most energy efficient) network possible. 3G is more efficient than 4G, and 2G or Edge uses less power than 3G.
There are AA chargers for most types of cell phones.
Unless you have an iPhone, which doesn't have a removal battery, consider getting an extra battery or, regardless of what phone you have, see if you can get your hands on a battery backup system like a AA charger from Energizer or Duracell or a rechargeable battery pack that works with your phone, though I realize it may be too late to do this before Sandy hits.
Consider charging your phone in the car, but don't let your car battery run down.
Also, if you have a car charger for your phone you might be able to use it if your house power fails. If you have a fully charged car battery you should still be able to do this and be able to start your car, but just to be safe you might want to run the engine for a few minutes to top off the battery, assuming you're not low on fuel and you're in a place where it's safe to run the engine.
If you have kids with cell phones, let them know that their phones are part of the family's emergency kit and that they, too, need to preserve battery life on their phones.
If you have a landline, it's best to have at least one corded phone. Cordless phones don't work if the power is out. A corded (not cordless) phone works if you have phone service but no power.
Cell towers could fail.
Even if you do all this, there is the possibility that you won't be able to use your phone because cellular base stations sometimes depend on wires to get data and power.